Emission after-treatment devices, or emission control devices (ECD), may be used to treat exhaust gas of internal combustion engines in order to reduce the amount of particulate emissions to atmosphere. In particular, emission control devices may include particulate filters (PF), oxidation catalysts, and nitrogen oxide (NOx) catalysts. Particulate matter, which is largely made up of carbon particles from incomplete combustion (e.g., soot), may collect in particulate filters, causing a gradual, increasing restriction of exhaust gas flow and reducing fuel economy as the particulate matter accumulates in the particulate filters. It will be appreciated that there are numerous types of particulate filters, including diesel particulate filters and gasoline particulate filters. In order to periodically purge or regenerate a particulate filter of the accumulated soot to reduce exhaust backpressure, measures may be taken that result in an increase in the exhaust gas temperature above a predetermined level to regeneration temperatures (e.g. above 450° C., for example) in order to incinerate the carbon particles accumulated in the filter to form gaseous products. In addition to soot, however, the exhaust gas also carries incombustible solid material that may remain after a regeneration, referred to as ash, and which may remain trapped in the PF for the remainder of its useful life. Ash is derived primarily from lubricating oil entering the combustion chamber or exhaust ports. Other sources include corrosion from the exhaust manifold and debris from the upstream catalytic converter. As particulate matter (e.g., ash and soot) accumulates in the PF, exhaust backpressure may increase, adversely affecting fuel economy. Because the stored ash may remain within the PF after regeneration, the exhaust backpressure created by the PF may be partially reduced as a result of regeneration, but may not be entirely eliminated. As such, the ash may continue to contribute to the exhaust backpressure on the engine, thereby reducing engine torque output and/or engine fuel economy.
Under some operating conditions, a vehicle may operate with exhaust gases reaching high enough temperatures to passively perform a particulate filter regeneration without selective or intrusive control of engine parameters to achieve the elevated exhaust gas temperatures for the purpose of particulate filter regeneration. In some instances, however, an active regeneration may be performed, where engine controls are adjusted in order to selectively increase the exhaust temperatures to facilitate particulate filter regeneration. Additionally, particulate filter regeneration may occur during deceleration fuel shut-off (DFSO) operating conditions. During DFSO operation, fuel injection to one or more cylinders is disabled during select operating conditions, such as a tip-out when coasting the vehicle down a hill, in order to reduce fuel consumption and increase fuel economy. The engine continues to rotate during DFSO, and thus air may still flow through the engine, to the exhaust, during DFSO operation, thereby increasing an oxygen concentration of gases flowing to the particulate filter. The increased oxygen concentration in the exhaust gas may promote a particulate filter regeneration for a particulate filter at regeneration temperatures.
Attempts to manage particulate filter regeneration conditions, including a temperature of the particulate filter, during DFSO include adjusting a length of a deceleration fuel shut-off event and a total number of activated and deactivated cylinders during the DFSO based on a particulate filter temperature change during a particulate filter regeneration. Further, fuel may be injected into one or more cylinders during DFSO in order to decrease the oxygen concentration of the exhaust gas flowing to the particulate filters (e.g., firing one or more cylinders at stoichiometry). Still further, the firing cylinders may be operated at a variable air/fuel ratio (e.g., lambda greater than, less than, or equal to 1). By combusting the air mixture, oxygen is consumed and the relative percent oxygen of the exhaust gas is significantly decreased, ultimately decreasing a rate of increase for the particulate filter temperature. One example approach is shown by Ulrey et al. in U.S. Application 2016/0222898. Therein, Ulrey determines whether a particulate matter reaction length (e.g., particulate filter regeneration) is greater than the length of DFSO, then one or more cylinders of an engine may be activated during DFSO to reduce an oxygen flow and extend a length of DFSO to match the reaction rate of soot. In this way, the oxygen flow rate is decreased while still performing a particulate filter regeneration during DFSO. By doing this, the filter may not exceed a maximum allowable particulate filter temperature, thereby reducing the likelihood of degradation of the particulate filter while completing the particulate filter regeneration.
However, the inventors herein have recognized potential issues with such systems. As one example, indiscriminately routing exhaust gas through the particulate filter, even under operating conditions when no net gain in emissions reductions or fuel economy is achieved, unnecessarily shortens the life of the particulate filter. Further, because DFSO may be controlled based on the temperature of the particulate filter when exhaust gas is routed through the particulate filter, this may disadvantageously limit the use of DFSO, thereby limiting opportunities to further increase fuel economy and reduce emissions by operating in a DFSO mode.
In one example, the issues described above may be addressed by a method for an engine, comprising: responsive to decreased soot generation or decreased soot storage, flowing gasoline combustion exhaust gas to a particulate filter with increased filter bypass flow even when an exhaust temperature is above a first threshold; and responsive to increased soot generation or increased soot storage, reducing the filter bypass flow and terminating deceleration fuel shut-off operation after a threshold duration due to exhaust temperature being above the first threshold. In some examples, the threshold duration may be based on one or more of the amount of soot generation, the amount of and soot storage, and the exhaust temperature relative to respective thresholds. When the filter bypass flow is increased, the engine may be operated in DFSO for a first duration based on an operator-demanded torque rather than a second duration based on a temperature of the particulate filter.
In this way, under operating conditions when the particulate matter load in exhaust gas is lower, a portion of exhaust may bypass the particulate filter, reducing exhaust back pressure and thereby improving fuel economy. Furthermore, by not flowing exhaust gas through the particulate filter during all driving conditions, soot and ash accumulation in the particulate filter may be reduced, thereby increasing the life of the particulate filter. Additionally, by allowing exhaust gas to bypass the particulate filter, the likelihood of excessive exotherms across the particulate filter during DFSO mode is reduced, and DFSO may be performed without regard for the temperature of the particulate filter. When particulate filter regeneration is desired, or when the exhaust gas contains an increased amount of soot, flow through the bypass may be decreased by decreasing an opening of a bypass valve disposed in the bypass. If the particulate filter is at an elevated temperature when DFSO is initiated, engine operating parameters and/or the DFSO operation itself may be adjusted in order to control the flow of oxygen-laden exhaust gas through the particulate filter. By controlling concentration of oxygen in the exhaust entering the particulate filter, a temperature of the particulate filter may be maintained below an upper threshold (e.g., a maximum allowed particulate filter temperature to reduce filter degradation) and thus, particulate filter degradation may be decreased while still performing regeneration and DFSO. As a result, the filter may not exceed the maximum allowed particulate filter temperature, thereby reducing the likelihood of a particulate filter degradation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.